Process for making a ceramic hot gas filter

ABSTRACT

A ceramic hot-gas candle filter having a porous support of filament-wound oxide ceramic yarn at least partially surrounded by a porous refractory oxide ceramic matrix, and a membrane layer on the outside surface of the porous support. The membrane layer is formed of an ordered arrangement of continuous filament oxide ceramic yarn which is at least partially surrounded by a porous refractory oxide ceramic matrix. The filter can withstand thermal cycling during backpulse cleaning and is resistant to chemical degradation at high temperatures.

This is a continuation of application Ser. No. 08/489,623 filed Jun. 9,1995, now abandoned, which is a division of application Ser. No.08/221,139, now U.S. Pat. No. 5,460,637.

BACKGROUND OF THE INVENTION

1. Field of the invention

The present invention relates to a composite ceramic candle filter forremoving particulates from a hot gas stream, and a method for makingsaid filter.

2. Description of Related Art

Ceramic filters have been tested in processes such as coal gasificationand coal combustion to remove particulates from hot flue gases toprotect downstream equipment from corrosion and erosion and to complywith EPA NSPS (New Source Performance Standards) regulations. Ceramicfilters in a tubular (candle) form, with one end closed and the otherend open have been shown to remove the particulates efficiently. The hotgas to be filtered typically flows from the outside to the inside of thefilter, with particulate-free gas exiting from the open end. The candlegeometry is also suited for removal of the filtered cake by backpulsingwith compressed gases.

Ceramic hot-gas candle filters must withstand exposure to chemicallycorrosive gas streams at temperatures in excess of 800 degrees C. Inaddition, they are subjected to significant thermal stresses duringbackpulse cleaning which can cause catastrophic failure of the ceramiccandle filter element.

Ceramic hot-gas candle filters known in the art are generally fabricatedfrom either porous monolithic materials or porous ceramicfiber-containing composite materials. Monolithic ceramic candle filtersare either weak or can fail catastrophically in use. Composite filtersare less susceptible to catastrophic failure and generally have improvedstrength, toughness, and thermal shock resistance versus monolithicceramic filters.

Candle filters may have relatively uniform porosity throughout thefilter or they may comprise a porous support with a thin layer, ormembrane, of fine porosity on the outer surface of the support. Themembrane layer is typically applied to the filter using a variety ofmethods such as coating from a dispersion containing finer grains thanthose used in the support for smaller membrane pore sizes, bondingrandomly arranged chopped ceramic fibers to the support using colloidal(or sol) materials, or forming a ceramic matrix by chemical vaporinfiltration.

Materials used to fabricate ceramic hot-gas filters generally includeoxides such as aluminosilicates, glass, and alumina, and non-oxides suchas silicon carbide and silicon nitride. Oxide-based ceramic filters haveadequate resistance to flue gas atmospheres and fly-ash for the designlife of the filters however they generally have low thermal shockresistance. Non-oxide ceramics generally have good thermal shockresistance, however they are susceptible to oxidation in the corrosiveenvironment to which they are subjected which results in a degradationof mechanical properties.

The disadvantages of ceramic candle filters known in the art includefailure, often catastrophic, due to thermally induced stresses caused bybackpulse cleaning, chemical degradation caused by species present inthe hot gases being filtered, delamination of the membrane layer,incomplete removal of the filter cake upon backpulsing, and high cost.They also tend to be heavy, requiring expensive support structures tohold an array of the candles in the filter unit.

The present invention provides a strong, lightweight ceramic hot-gascandle filter which has a greater than 99.5% particulate collectionefficiency, thus meeting EPA NSPS regulations. The filter of the presentinvention comprises a filament wound support having a membrane layerapplied to the outer surface thereof. The support comprises areticulated tube of yarns comprising ceramic oxide fibers which are atleast partially coated with a porous refractory oxide matrix. Themembrane layer comprises an ordered arrangement of continuous filamentceramic oxide yarns which are also coated with a porous refractory oxidematrix. The membrane layer is firmly adherent to the support andtherefore does not suffer from delamination problems. The porosities ofthe support and membrane are controlled such that the support functionsas a bulk filter and the membrane layer functions as a surface filter.Failure of the filter is generally not catastrophic since if themembrane is damaged, the support quickly blinds at the location of thedamage due to its bulk filtration properties, thus preventing release ofparticulates and protecting downstream process equipment such as gasturbines or sorbent beds. The filter of the present invention isresistant to chemical degradation due to the oxide compositions used,and at the same time provides excellent thermal shock resistance whichis not generally typical of oxide materials. The smoothness of themembrane surface results in efficient removal of the filter cake duringbackpulse cleaning. In addition to the above-mentioned advantages, thefilter of the current invention is potentially low cost relative to mostof the commercially available candle filters.

SUMMARY OF THE INVENTION

The present invention is directed to a ceramic hot gas filter comprisinga porous elongated filter support and a porous membrane layer on theouter surface of the support. The support has an opening at one end intoa hollow interior, a closed end opposite the open end, and an externalflange integral with the open end. The support is formed of a pluralityof layers of oxide ceramic support yarn, each layer being arranged in acriss-crossing relationship with neighboring layers to form a pluralityof quadrilateral-shaped openings. The yarn in the support is coated witha first oxide ceramic material which, upon heat treatment, forms aporous refractory oxide support matrix. The membrane layer is formed ofan ordered arrangement of continuous filament oxide ceramic membraneyarn. The yarn in the membrane is coated with a second oxide ceramicmaterial which, upon heat treatment forms a porous refractory oxidemembrane matrix. Preferably, the support yarn and the continuousfilament membrane yarn each contain at least 20 weight percent alumina(Al₂ O₃) and have softening points above about 750 degrees C. The oxideceramic coating materials are generally particulates of oxides or oxidecompounds, or mixtures thereof and may also include oxide precursormaterials. Methods for forming the membrane layer include hoop winding,multiple yarn winding, and fabric wrapping. The membrane layer has aporosity that is less than that of the support. Preferably thequadrilateral-shaped openings have dimensions of about 100 to about 500microns after heat treatment so that the support functions as a bulkfilter. The membrane layer preferably has pore diameters of about 0.1 to10 microns and functions as a surface filter. In a preferred embodimentof the invention, the support yarn has generally the same composition asthe membrane yarn and the support matrix has generally the samecomposition as the membrane matrix.

The present invention also provides a method for making a ceramic hotgas filter involving the steps of fabricating an elongated porous filtersupport by coating a ceramic oxide support yarn with a first coatingcomposition, winding the coated support yarn onto a mandrel to form aplurality of layers of the coated support yarn, each layer beingarranged in a crisscrossing relationship with neighboring layers to forma plurality of quadrilateral-shaped openings. The mandrel is contouredto provide an integral external flange adjacent one end of the support.The resulting support has an open end adjacent the flange, an outsidesurface, and a second open end opposite the flanged end. A membranelayer is formed on the outside surface of the support by coating acontinuous filament oxide ceramic membrane yarn with a second coatingcomposition and applying the coated membrane yarn in an orderedarrangement on the outer surface of the support. The second open end isclosed using an oxide ceramic material. In a final step, the support andmembrane layer are heat treated to convert the first coating compositionto a porous refractory oxide support matrix and to convert the secondcoating composition to a porous refractory oxide membrane matrix. Thefirst and second coating compositions are preferably slurries ofparticulates of oxides or oxide compounds, or mixtures thereof and mayalso include oxide precursors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic perspective view of an embodiment of a filterelement of the current invention, including an optional flange collarsection.

FIG. 1B is a cross section of the filter element taken on line 1B--1B ofFIG. 1A.

FIG. 1C is a cross section of the flange section taken on line 1C--1C ofFIG. 1A.

FIG. 1D is a cross section of the flange section taken on line 1D--1D ofFIG. 1A.

FIG. 1E is a cross section of the closed end taken on line 1E--1E of FIG1A.

FIG. 2 shows openings formed by the overlap of two layers of yarn in asupport layer comprising an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The hot-gas filter of the current invention is of the candle filter typeand comprises a porous ceramic support having a porous ceramic membranelayer on the outer surface thereof. The support has good filtrationcapacity for fly-ash and serves as a bulk filter, capable of trappingparticulates between its inner and outer surface. The membrane serves asa surface filter, preventing particulates from penetrating therethrough.

By the term "ceramic" is meant crystalline or partially crystallinematerials, or non-crystalline glasses, which comprise essentiallyinorganic, nonmetallic substances. Ceramics useful in the currentinvention are generally oxide ceramic materials and are essentially freeof non-oxide ceramics so that the filter material is generally stable athigh temperatures in corrosive environments. The term "oxide" is meantto include oxides, oxide compounds (e.g. mullite, spinel), or precursorsthereof. The term "refractory" is meant to include ceramic materialshaving a melting point of at least 1000 degrees C.

Referring to FIGS. 1A-1E, the filter 10 comprises a support 12 having agenerally elongated tubular shape with an open end 14 at one end into ahollow interior. The end 15 of the support opposite the open end isgenerally closed. The support further includes an external flange 16integral with the open end 14 which supports the filter in a tube sheetin use. The flange may also include an optional collar insert 24,integral with the flange, and described in more detail below. Themembrane layer 18 is formed on the outer surface 20 of the support. End15 is generally closed by filling with a ceramic material 26, and theflange section 16 and tip section of the support adjacent the closed end15 made impervious as described below.

The porous support and membrane layers comprise ceramic oxide yarnswhich are at least partially surrounded by a porous refractory oxidematrix. The yarns are laid down in such a fashion as to obtain amembrane layer having a porosity which is less than the porosity of thesupport layer.

The overall porosity of the support layer is determined by a combinationof the open volume created by the diamond or parallelogram-shapedopenings (macropores) and the porosity of the support matrix(micropores). The porosity of the membrane layer is due to any defects(i.e. gaps between the coated membrane yarns) in the membrane layer aswell as the porosity of the membrane matrix (micropores).

The macroporosity of the support may be calculated from the volume ofthe support (calculated from the measured dimensions of the support),the weight of the support, and the bulk density of the support (fiberand matrix, including any microporosity). The bulk density is measuredusing mercury porosimetry.

The matrix is applied in such a way that the channels in the support arenot substantially closed. The matrix generally imparts integrity andmechanical strength to the support and also provides an excellent degreeof thermal shock resistance because of the ability of the porous matrixto absorb thermally induced mechanical stresses which might otherwisefracture the fibers in the filter.

The support is formed of a plurality of layers of continuous ceramicoxide yarns which are laid down in spaced helical coils in acriss-crossing relationship with neighboring layers to form a pluralityof diamond or quadrilateral-shaped openings having dimensions between100 and 500 microns after firing. The openings form channels extendingbetween the inner 22 and outer 20 surfaces of the support which followtortuous, curved paths. If the filter is damaged, for example bydamaging the membrane layer during installation, it will quickly"self-heal" by functioning as a bulk filter and blinding withparticulates in the hot gas stream. A support containing a significantnumber of straight radial channels will not blind as readily, resultingin failure of the filter. Forsythe, U.S. Pat. No. 5,192,597,incorporated herein by reference, describes filament winding ofreticulated ceramic tubes in a preferred winding pattern. The yarns inadjacent layers of diamond-like patterns are laid down in such a mannerthat the yarns forming the walls of the diamonds of each layersubstantially cover the diamond shaped openings of each adjacent layer.This forms a tubular structure comprising series of interconnecteddiamond shaped openings, each layer of which interfere with the directflow of gas from one layer to the next.

The winding pattern described above is for the elongated central bodysection of the support (i.e. the generally cylindrical section of thefilter between the flange and closed end). Due to the contoured closedend and flange sections of the filter, the described winding pattern isnot achieved at the flange and closed end.

FIG. 2 shows two adjacent layers of yarn in a support prepared accordingto U.S. Pat. 5,192,597 (the matrix layer is not shown in this Figure)which define openings designated by "x". The size of the openings iscontrolled by the spacing between the yarns in each layer which isdetermined by the wind angle and yarn denier in addition to the amountof matrix material applied to the yarn. The spacing "a" between adjacentyarns is preferably controlled to provide openings having dimensions "a"of between about 100 and 500 microns in the final support, after hightemperature firing. The openings have larger dimensions near the innersurface of the support, with the dimensions gradually decreasing insize, as winding continues, to the outer surface. The dimension "a" canbe calculated based on the yarn spacing and the amount of matrix appliedto the yarn. Alternately, "a" can be measured visually in the finalsupport. A support having the described construction and having openingsin this size range will function as a bulk filter which can trapparticulates within the wall of the support while maintaining a pressuredrop that is insignificant relative to the pressure drop across themembrane layer.

The support may be formed by winding a ceramic oxide yarn on a suitablydesigned mandrel using a filament winder designed to maintain a constantwinding ratio (rotational speed of the mandrel divided by the speed ofthe traverse arm). A constant winding ratio is necessary to maintain theproper size and distribution of channels throughout the wall. The flangesection of the support is formed by using a mandrel that is wider at oneend, the wide end being contoured to give the desired flange geometry.Filament winding on such a mandrel produces a tube with an externalflange section at the open end and a small hole at the opposite end,which is generally closed in the final support, as shown in FIG. 1E,with a ceramic material 26. Alternately, if it is desired that theinside wall of the support be straight as opposed to contoured at theflange section, a filament wound collar insert 24, shown in FIG. 1C andFIG. 1D, having a composition similar to that of the support and havingan inner diameter approximately equal to the outer diameter of themandrel and an outer surface contoured to give the appropriate flangegeometry may be used. The collar is inserted on the mandrel and thesupport is then wound on the combined mandrel and collar. When thesupport is removed from the mandrel, at least a portion of the collarremains with the support as part of the flange section, as illustratedin Example 2 below.

Field tests have demonstrated that hot-gas candle filters commonly failat the flange section. According to the current invention, the flangeand the body of the support are formed as a single unit to ensurehomogeneity of the support material across the entire filter and toeliminate any stresses or weak spots arising from joining materials. Theshape of the flange is not critical but should be reproducible. Theflange should provide a good seal with the tubesheet that supports thefilter in use so that no dust leakage occurs. The shape of the closedend is generally round, but various shapes are possible by suitablyshaping the mandrel. The diameter of the opening at the closed end ofthe tube depends on the thickness of the shaft that supports themandrel.

The membrane layer is applied to the outer surface of the support andcomprises an ordered arrangement of continuous filament oxide ceramicyarns that are at least partially surrounded by a refractory oxidematrix. Preferably the membrane covers the essentially all of theoutside surface of the support, however it is not necessary for themembrane to be applied to the flange section of the support or to theend of the support which is later closed with a ceramic oxide cement oroxide slurry. The membrane layer in the final filter, after heattreatment, has pore diameters of between about 0.1-10 microns,preferably 0.1-5 microns.

The ordered arrangement of yarns in the membrane layer may be formed byvarious methods including circular (hoop) winding, multiple yarnwinding, or wrapping with yarns pre-arranged in two or three dimensionalforms such as fabric or braided materials. The membrane yarns should bearranged so as to obtain a smooth outer membrane surface. A smoothmembrane surface is desirable because it facilitates complete removal ofthe filtered material during backpulse cleaning because the filter cakereadily debonds from the smooth surface. If the surface is rough, thefiltered cake tends to be mechanically anchored to the surface making itdifficult to completely remove the cake by backpulse cleaning. Thesmoothness of the membrane layer depends on the number of windings perunit length and the diameter of the sizing orifice, in addition to theyarn construction.

The yarns used to form the support and membrane layer preferablycomprise ceramic fibers having softening points of at least about 750degrees C., more preferably at least 1000 degrees C. The phrase"softening point" is used herein to mean both the softening point of aglass ceramic and the melting point of a crystalline ceramic. The yarnsused in the membrane layer may be the same as or different than theyarns used in the support.

Suitable oxide fibers include certain glass fibers such as S glass (hightensile strength glass containing about 24-26% alumina(Al₂ O₃)), "FiberFrax" aluminasilicate fiber, and polycrystalline refractory oxide fiberscontaining at least about 20% by weight of alumina such as thealumina-silica fibers disclosed in Blaze U.S. Pat. No. 3,503,765 andcertain of the high alumina content fibers disclosed in Seufert andD'Ambrosio U.S. Pat. No. 3,808,015and U.S. Pat. No. 3,853,688.Preferably the oxide fibers comprise between 20% and 80% by weight ofaluminum oxide. Examples of commercially available aluminosilicatefibers include "Altex" (Sumitomo) and "Nextel" (3M) fibers. Fiberscontaining significant levels of glass-forming oxides such as B₂ O₃ andP₂ O₅ are not desirable because they will flux the entire structureresulting in a dense, nonporous support.

Fibers of refractory oxide precursors can also be used to form thesupport. After winding, the precursor fibers are converted topolycrystalline refractory oxide fibers by firing to remove volatiles,convert salts to oxides, and crystallize the fiber. The preparation ofrefractory oxide fibers and their precursors is disclosed in U.S. Pat.Nos. 3,808,015 and 3,853,688.

The oxide fibers generally have diameters in the range of 0.2 to 2.0mils (0.005-0.05 mm) and are used in the form of continuous yarns,preferably containing 10=14 2,000 or more fibers. The fibers arepreferably continuous filaments, however yarns of staple fibers can beused, especially glass. The yarns are preferably loosely twisted so thatany loose or broken ends do not interfere during filament winding whenthe yarn is pulled through small orifices. The yarns may also be used inthe form of roving. Bulked, interlaced, or textured yarns may be used.However, the yarns used in the membrane layer most preferably comprisecontinuous filament, untextured yarns so as to obtain a membrane layerhaving a smooth outer surface. Glass yarns which crystallize to formrefractory oxides upon high-temperature heat treatment are preferredbecause they are easier to handle and less likely to break duringfilament winding than yarns containing crystalline ceramic fibers.

The refractory oxide matrix components of the support and membranepreferably have softening points above 1000 degrees C., more preferablyabove about 1400 degrees C., and most preferably above 2000 degrees C.Preferably the matrix comprises at least 40 wt % alumina.

The matrix components are generally applied to the support and membranelayer in the form of a coating composition which is then fired to form arefractory oxide matrix. The coating composition used in the support maybe the same as or different than the coating composition used in themembrane. The coating composition generally comprises an aqueoussolution, suspension, dispersion, slurry, emulsion, or the like whichcontains one or more oxide particulates or oxide precursors. Preferablythe oxide particulates have a particle size of 1-20 microns, morepreferably 1-10 microns, most preferably between 1-5 microns. Particlesizes less than 20 microns are preferred because they are readilydispersed and penetrate into voids between fibers. Slurries preparedusing particle sizes less than 1 micron are generally too viscous atuseful solid concentrations. Oxide particulates useful as matrixmaterials include alumina, zirconia, magnesia, mullite, spinel. Suitablematrix precursors include water soluble salts of aluminum, magnesium,zirconium and calcium such as "Chlorhydrol" (aluminum chlorohydratesolution sold by Reheis Chemical Co.), zirconyl acetate, aluminahydrate, basic aluminum chloracetate, aluminum chloride, and magnesiumacetate.

Preferably, drying control additives such as glycerol and formamide areadded to the coating composition at levels of 1-5 wt % based on thetotal weight of the coating composition. The drying control additivesreduce drying stresses in the green body and also eliminate macroscopiccracks on the surface of the high-temperature fired filter.

The coating composition preferably includes a ceramic oxide binder toincrease the green strength of the wound structure. The soluble oxideprecursors which are useful as matrix precursors also function asbinders. A preferred binder is fumed alumina, which contains aluminaparticles having particle sizes of about 10-50 nm. Preferably thecoating composition includes between about 10-25 wt % of fumed alumina,calculated based on the total solids content of the coating composition.The fumed alumina serves to bond the larger oxide particulates togetherand increases the green strength of the support. The binders areincorporated into the refractory matrix upon heat treatment.

The coating composition may be applied to the support by drawing theceramic oxide yarn through the coating composition prior to winding on amandrel. Preferably, the coating composition is uniformly distributedaround the fibers of the yarn. The distribution is affected by theviscosity of the coating composition, the method of application, thedensity (or tightness) of the yarn bundle, the nature of the yarn andthe amount of the coating composition. The composition should have aviscosity that is low enough to permit flow and some penetration intovoids in the yarn but high enough to adhere to the yarn bundle. When thecoating composition is a particulate slurry, the solids content ispreferably between 50-75 wt % and the slurry preferably has a viscosityin the range of 100-300 centipoise. If a coating composition containingboth an oxide precursor and particulate oxide powder is used, the solidscontent of the slurry should be adjusted to about 60-90 wt % of therefractory oxide matrix material derived from the oxide particulate andabout 10-40 wt % derived from the precursor. It is difficult to obtainsufficient amounts of oxide-containing materials in the coatingcomposition using levels of precursor greater than about 40 wt %. Theamount of matrix material applied to the yarn can be controlled bypulling the yarn through a suitably sized die to remove excess slurry.The coating composition may be also be applied to the yarn by use of afinish roll, spraying, etc.

Alternately, the matrix coating composition may be applied to the woundsupport by dipping the support in a slurry, draining off the excess anddrying. Additional dipping steps may be used if necessary to provide thedesired weight of matrix relative to the weight of yarn in the support.In general, it is difficult to apply the matrix coating composition bydipping without closing a significant portion of the channels in thesupport, which is not desirable and results in increased backpressure.

The membrane matrix coating composition may be applied to the membranelayer using methods similar to those described for the support.Preferably the combined weight of the matrix components of the supportand membrane layers comprises about 40-70% of the final weight of thefilter, more preferably about 50-60%. To avoid thermal stresses, it ispreferable that the support yarn has generally the same composition asthe membrane yarn and the support matrix has generally the samecomposition as the membrane matrix. For the same reason, it ispreferable to have a weight ratio of fiber to matrix which isessentially the same in the membrane and the support.

In a preferred embodiment, the membrane is formed by hoop winding. Theoxide ceramic membrane yarn is coated with the membrane matrix coatingcomposition, for example by passing through a bath containing a coatingcomposition, followed by passing through a sizing orifice to removeexcels slurry, and winding tightly at approximately 90 degrees to theaxis of mandrel. Preferably, the diameter of the sizing orifice iscarefully selected to give a matrix pick-up that yields similar weightratios of fiber and matrix in the membrane and support layers. The rateof mandrel rotation relative to the rate of the movement of thetransverse arm is controlled so that the yarns are laid down adjacenteach other as close as possible with substantially no overlapping ofyarns or gaps between yarns in the membrane layer. A proper selection ofthe number of windings per unit length will give a continuous and smoothmembrane surface.

Alternately, multiple yarns are combined and wound on the support atsubstantially the same wind angle as that of the support to fill theunderlying openings in the support. This may be accomplished by feedingthe separate yarns through tensioning devices, dipping in a ceramicmatrix particulate slurry, and combining the yarns just prior to pullingthrough a larger sizing orifice than that used for single yarn ends andwinding on the support. The diameter of the sizing orifice is selectedas described above for hoop winding.

A membrane comprising a single filament wound layer on the support isgenerally adequate for many filtration applications. Additional layersof wound yarns may be applied to increase the thickness of the membranelayer. This usually increases the particulate collection efficiency andthe back pressure of the filters.

In a third embodiment, the membrane layer is formed by wrapping thesupport with a ceramic fabric. The fabric is wrapped on the filtersupport and a matrix slurry similar in composition to that used in thesupport is brushed on the fabric. The slurry wets the fabric and thesupport, and provides bonding to the support. Any wrinkles in the fabricare removed while still wet. Additional layers of fabric are wrapped onthe support as necessary to increase the filtration efficiency. Thefabrics useful for building the membrane layer include tightly wovenplain and satin weaves. It may be necessary to use a matrix slurrycontaining matrix particulates having a smaller particle size than thematrix particulates used to wind the support in order to improve theadhesion between the filter support and fabric membrane layer. This isbecause the smaller particles will more readily infiltrate theinterstices in the woven fabric. In general, this method is lesspreferred because it is more difficult to control the amount of matrixapplied to the membrane layer. In addition, it has been found that thefabric layers tend to be less strongly adhered to the support thanmembranes formed using the filament winding techniques described above.

The flange section and the closed end may be reinforced and madeimpervious to any gas streams by saturating with a ceramic slurry orusing a ceramic cement composition. To avoid reactions with theunderlying support material and to match the thermal expansion of thesupport, the matrix material used in the support is preferred for thispurpose. Closing of the bottom may be accomplished using commercial hightemperature cements or by filling with a wad of the ceramic yarn used toform the support, dipping in a ceramic matrix particulate slurry, andfiring. The cement may have a higher solid content than the matrixcoating composition and may contain dispersed ceramic fibers for higherstrength. The solids in the cement should not react with the tubematerial, which would reduce the thermal stability of the filter. Priorto application of the slurry or cement composition, the structure shouldbe fired at temperatures between about 700 degrees C. and 1400 degreesC. in order to stabilize the structure so that it does not deform whencontacted with the slurry, cement, or other materials in laterprocessing steps. If this initial firing is carried out at temperaturesless than about 1200 degrees C., the structure will require a hightemperature firing step at 1200 to 1400 degrees C. to form stablecrystalline phases and to stabilize the added material after applicationof the slurry or cement. If the initial firing is carried out at 1200 to1400 degrees C., an additional low-temperature firing at between about700 degrees C. and 1000 degrees C. is necessary after application of theslurry or cement to stabilize the added material.

The green candle filter is generally dried at room temperature until itis strong enough to handle. It may then be fired at temperatures belowthe softening point of the ceramic yarn and sufficiently above theboiling point of any volatiles, typically around 300 to 800 degrees C.,to remove the volatiles and stabilize the filter. This is especiallyimportant when oxide precursors are used. An additional firing at hightemperatures is then carried out, typically at 1200 to 1400 degrees C.,to form stable crystalline phases. Firing above 1450 degrees C. may meltsome of the phases and result in a fused product which is undesirabledue to reduced thermo-mechanical properties. Preferably, the heatingrate during the high temperature firing does not exceed 20 degrees C.per minute, in order to allow any glass phases to crystallize, and maybe as low as 0.1 degrees C. per minute. During high temperature firing,glass fibers may devitrify into crystalline phases, the matrix mayconvert to stable crystalline phases or the crystalline phases in thefiber and matrix may react to form new stable crystalline phases. Thefinal phase composition of the product depends on the amounts of fiberand matrix, the heating profile, soaking time at intermediatetemperatures and the dwell time at the highest firing temperature. Thetypical crystalline phases are corundum, mullite, cordierite andcristobalite. As used herein, the term cordierite is intended to includeindialite, a crystalline material having the same composition ascordierite, but a slightly disordered crystal structure. Excesscristobalite formation is undesirable since cristobalite undergoes avolume change at 200-270 degrees C., which contributes to poor thermalshock resistance. The final filter should contain no more than 10% byweight cristobalite. Preferably the final composition of the filter is3-7 parts by weight magnesia, 20-45 parts silica and 45-70 partsalumina. More preferably the final filter comprises between about60%-70% alumina.

In a preferred embodiment, the yarn used to prepare both the support andmembrane comprises glass fibers comprising 61-66% SiO₂, 24-26% Al₂ O₃,and 9-10% MgO. A coating composition consisting essentially of aluminais applied to the yarn prior to winding in an amount sufficient toprovide a refractory oxide matrix comprising 40-70% of the final weightof the filter. The coating composition contains a binder comprisingfumed alumina particulates having an average particle size of 13-15 nmand alumina matrix particulates having an average particle size of 2-3microns. The membrane is applied to the support by hoop winding. Thegreen filter element is heated to remove volatiles and then hightemperature fired at temperatures above about 1350 degrees C.,preferably at a temperature of about 1380 degrees C. During hightemperature firing, the glass fiber softens and the silica and magnesiain the glass in combination with the alumina matrix material combine toform cordierite and mullite. The final filter comprises about 20-40% byweight SiO₂, about 3-6% by weight MgO and about 50-70% by weight Al₂ O₃.The final crystalline composition, after heat treatment, is 25-40%cordierite, 5-15% mullite, 40-60% corundum and 0-10% cristobalite, basedon the total crystalline content. Approximately 50-90 vol % of thematerial is crystalline with the remainder being amorphous. Theformation of crystals of mullite, cordierite, and corundum, each havingdifferent coefficients of thermal expansion, leads to formation ofmicrocracking in the structure. The microcracks form along crystallineboundaries as well as within regions having only a single crystal phase.The microcracks are believed to absorb stresses caused by thermal shock.After firing, the filter is stable up to 1200 degrees C. for extendedperiods of time and has excellent thermal shock resistance.

EXAMPLES

All percentages referred to herein are weight percent, unless otherwiseindicated.

The filament winder Used to wind the support in the Examples below had achain-driven traverse of approximately 70 inches (178 cm) (278 teeth of0.5 inch (1.27 cm) pitch passing in a narrow loop driven and supportedby 11 tooth drive sprockets at each end). The drive ratio was set suchthat the spindle rotated at a speed of 50 and 10/111 revolutions foreach complete rotation of the chain loop for winding of the filtersupport. The mandrel was a tube having a length of 65 inches (165 cm)and an outer diameter of 1.75 inches (4.45 cm) with end closures at eachend. One of the end closures was conical with about a 30 degree taper oneach side of the cone with a 0.50 inch (1.27 cm) diameter drive shaftmounted at its axis. The second end closure was hemispherical (1.75 in(4.45 cm) diameter) with a 0.25 inch (0.64 cm) drive shaft mounted atits axis. The mandrel was attached to and driven by the spindle in sucha position as to be traversed along its length by the traversing yarnguide. The mandrel was attached to and driven by the spindle via the0.50 inch (1.27 cm) shaft and supported in a bearing at the 0.25 inch(0.64 cm) shaft. It was mounted parallel to the chain-driven traverseguide such that the guide traversed above the mandrel surface at adistance of about 0.75 inch (1.91 cm) from the surface of the mandreland the traverse stroke extended from about 0.75 inch (1.91 cm) past thehemispherical closure onto the 0.25 inch (0.64 cm) shaft and to about0.75 inch (1.91 cm) past the conical closure onto the 0.5 inch (1.27 cm)shaft.

A plastic collar having a 7 mm wall thickness and a 45 degree edgerelative to the axis of the collar was inserted on the mandrel near theconical end to form the flange on the filter support for Examples 1 and3.

For Example 2, a separate winder having a 6 inch (15.2 cm) traversestroke with means to adjust this stroke to contour the package ends wasused to form a collar insert for the flange section of the filter. Thedrive ratio was set such that the spindle rotated at a speed of 4 and11/180 revolutions for each complete rotation of the traverse cam toprovide the same wind angle in the collar insert as the wind angle inthe support. A mandrel comprising a short piece of 1.75 inch (4.45 cm)outer diameter tube was mounted on the spindle and wrapped with 2 layersof 0,002 inch (0,005 cm) thick "Mylar" polyester film to facilitateremoval of the wound unit. The mandrel was wrapped with 90 grams ofS-glass (S-2 CG150 1/2 636, available from Owens-Corning FiberglasCorporation of Toledo, Ohio) that was coated with an aqueous A-17alumina slurry (see Example 1for composition of slurry) applied in sucha quantity to form a unit having 50-60 wt % ceramic from the slurry and40-50 wt % ceramic derived from the feed yarn after drying. The collarinsert, as wound, had the form of a cylinder of approximately 1.75 inch(4.45 cm) inner diameter and a 3/8 inch (0.95 cm) wall thickness withthe ends of the cylinder wall exhibiting a taper of approximately 45°.The insert was removed from its mandrel while still wet and transferredto the mandrel on the main filament winder, described above. The insertwas positioned so as to leave about 57 inches (145 cm) of the straighttube portion of the mandrel exposed between the insert edge and thejuncture of the tube with the hemispherical end closure.

The filter support units were wound onto the mandrels with either thecollar insert or plastic collar mounted thereon. Winding was carried outwith the spindle set at a rotational speed of approximately 500-520revolutions per minute. The final (fired) support units haddiamond-shaped openings on the outer surface having dimensions of about175-250 microns.

TEST METHODS

The density and porosity of the membrane layers was determined usingmercury porosimetry. Membrane samples were prepared for porosimetrymeasurements using either of two methods. The membrane layer can bereadily debonded from the support prior to firing of the candleassembly. The debonded membrane layer is then high-temperature fired andsubmitted for porosimetry measurements. Alternatively, the membranesample may be prepared by scraping away the support layer from a sampleof a high-temperature fired candle assembly. The median pore size isreported in microns and the porosity is reported in volume percent. Themedian pore size is the value obtained at the maximum intrusion volume.

The average oxide composition was determined using X-ray Fluorescencespectroscopy. The samples and standards were fused in a lithiumtetraborate flux and the X-ray emission lines for the elements ofinterest were measured. The results are reported as weight percent withthe samples being dried at 130 degrees C.

Crystalline phase compositions were determined using X-ray diffractionusing a Scintag Pad X theta-theta diffractometer using Cu K-alpharadiation. The following conditions were used: copper tube operated at45 kilovolts, 40 milliamps, goniometer radius 250 mm, beam divergence0.24degrees, scatter slit 0.43 degrees, receiving slit 0.2 mm, germaniumsolid state detector bias 1000 V, scan speed 0.2 degrees 2-theta perminute, chopper increment 0.03 degrees 2-theta, scan range 3 to 112degrees 2-theta (overnight scans), samples front packed against filterpaper in a 1 inch square aluminum well-type sample holder, single samplechanger. The samples were wet milled in acetone for 5 minutes in aMcCrone vibratory mill using corundum grinding elements and dried undera heat lamp. The percentages of crystalline phases were determined basedon a mixture of standard materials with 20% fluorite as an internalstandard. Standard materials used were NIST (NBS) 674 alpha alumina(corundum), Baikowski high purity cordierite (indialite) standard, Coorsmullite standard, NIST (NBS) 1879 cristobalite, NIST (NBS) 1878 quartz,and Coors spinel standards. The samples themselves were not mixed withan internal standard but were normalized to 100% of the crystallinecomponents after dividing each measured intensity by its respectivereference intensity ratios. Analysis lines were: indialite at 10.4, 18.2and 29.5 degrees; mullite at 16.5 and 26.1 degrees, corundum at 25.6 and52.6 degrees, cristobalite at 21.8 degrees (overlap corrected forindialite), and quartz at 20.8 degrees.

Example 1

This example illustrates the fabrication of a ceramic filter accordingto the current invention, wherein the membrane layer is formed using awoven glass fabric.

An alumina slurry was prepared by charging 7.0 liters of water and 20.0ml of formic acid in a mixing vessel. Fumed alumina having an averageparticle size of 13-15 nm (manufactured and sold by Degussa) (2.0 kg)was added slowly with stirring. The pH of the dispersion was adjusted to4.0 to 4.1 using formic acid. After stabilizing at this pH for twohours, 11.0 kg of A-17 alumina (average particle size 2-3 microns,manufactured and sold by Alcoa) was added in portions and stirredovernight. Glycerol was added to the slurry at a level of 3 wt % basedon the total weight of the slurry. The solids content of the dispersionwas 62-65 weight percent and the viscosity was adjusted to 140centipoise by water addition, measured with a Brookfield viscometer(Model No. RV1) using the #1 spindle.

A 2-ply glass yarn (150 filaments/ply) comprising 65.2% SiO₂, 23.8% A1203, and 10.0% MgO having a hydrophilic sizing to aid wetting by theaqueous coating composition (S glass, designation S-2 CG150 1/2 636,available from Owens-Corning Fiberglass Corporation) was fed through aball tensioner, passed through the alumina slurry, and pulled outthrough a 0.017 in (0.043 cm) die to remove excess slurry. The diecontrolled the amount of slurry applied to the yarn so that, afterdrying, about 50-60% by weight of ceramic in the support was from theslurry and about 40-50% by weight was derived from the yarn. The wetyarn was then passed through a guide attached to the traverse arm of thefilament winding machine and wound onto the contoured mandrel describedabove wrapped with 2 layers of 0,002 in (0.005 cm) "Mylar" polyesterfilm. The winding was stopped after about 1000 grams of yarn were woundonto the mandrel, when the support reached the desired outside diameter(approximately 60 mm). After drying overnight at room temperature, thefilament-wound tube was removed from the mandrel by cutting through thewound material at about the center of the raised flange section(indicating the location of the plastic collar insert) and removing thetwo pieces from the opposite ends of the mandrel.

The membrane layer was attached to the support as follows. S-2 glassfabric (plain weave, 1.5 oz/square yard) available from Burlington GlassFabric (Altavista, Va.) was cut into pieces of length and widthapproximately equal to the length and circumference of the tuberespectively. Each piece was wrapped on the body of the tube and analumina slurry containing A-16 alumina (manufactured and sold by Alcoa,average particle size 0.45 micron) with 55 to 60 weight percent solidcontent, 3 wt % glycerol, and 100 to 120 cps viscosity, was brushed onthe fabric. The fabric was not applied to the flange and the bottom endof the tube. Any wrinkles in the fabric were removed by rubbing with awet sponge while the fabric was still wet before adding additionallayers of fabric. Two additional layers of fabric were attached in asimilar manner such that the closing of the ends in each layer of fabricfell approximately 120 degree apart in the final filter. After allfabric layers were applied, the tube was dried overnight at roomtemperature. It was then low-temperature fired at 700 degrees C. for onehour in a muffle furnace to remove volatiles and stabilize thestructure.

The flange section was reinforced and sealed by dipping one time in analumina slurry (fumed alumina/A-17 alumina, described above) anddraining off the excess. A wad of S-2 glass fibers was inserted into thehole in the bottom end of the filter and the bottom end was then dippedin the A-17 alumina slurry. After thorough drying and firing at 700degrees C. for one hour, the filter was fired in a high temperaturefurnace. The temperature was increased to 800 degrees C. in about 40minutes, held for about 20 minutes, then increased to 1300 degrees C. ata rate of 2 degrees C./minute, held for 2 hours, then heated at a rateof 1 degree C./minute to 1380 degrees C., held for two hours and cooledto 800 degrees C. at a rate of 5 degrees C./minute, followed byunrestrained cooling of the furnace to 200 degrees C. The filter wasthen removed from the furnace and allowed to cool to room temperature inair.

The membrane layer had a bulk density of 1.62 g/cc and a volume porosityof 39% with a median pore diameter of 0.45 μm, measured by mercuryporosimetry. The average oxide composition of the filter, determined byX-ray fluorescence, was 27% silica, 68% alumina and 4% magnesia. Thecrystalline phase composition, determined by X-ray diffraction, was 35%cordierite (indialite), 6% mullite, 50% corundum and 9% cristobalite.

Example 2

This example illustrates the fabrication of a ceramic filter of thecurrent invention, wherein the membrane layer is formed by circularwinding.

A filter support was prepared in a manner similar to that described inExample 1 except that the filament-wound collar insert was used to formthe flange section instead of the plastic collar. When the supportelement was cut through for removal from the mandrel, the wound collarwas cut through as well such that a section of the original collarremained in the flange section of the support.

The membrane was applied to the support by circular (hoop) winding of aglass yarn (S-2 CG 150 1/2 636, available from Owens-Corning FiberglasCorporation) on the surface of the support. The mandrel with the driedsupport wound thereon was transferred to a specialized winder forformation of the membrane layer. The filament winder used for formationof the membrane layer had a screw driven traverse, with the drive ratioset such that the spindle rotated at a speed of 75 complete revolutionsfor each 1 inch (2.54 cm) travel of the traverse guide so that the yarnwas placed at a spacing of 75 yarns per linear inch of tube surface.Adjacent yarn windings were as close to each other as possible withoutoverlapping. The yarn was soaked in the A-17/fumed alumina slurry, andpulled through a 0.017 in (0.043 cm) die prior to winding. About 60grams of yarn were wound on the support surface to form a single layerof winding over its length. The circular winding was done across theentire length of the filter, bottom end and flange section. Afterovernight drying (12-16 hours) at ambient temperature, the tube wasremoved from the mandrel as described in Example 1. After inspection fordefects, the filter unit was fired at 700 degrees C. for two hours. Thenthe bottom hole was then filled with a wad of S-glass yarn. The flangeand bottom sections of the tube were dipped in the A-17/fumed aluminaslurry, the excess drained off, and dried thoroughly. The combinedsupport and membrane was then high-temperature fired as described inExample 1.

The membrane layer had a bulk density of 1.61 g/cc and a volume porosityof 39% with a median pore diameter of 0.43 μm, as measured by mercuryporosimetry. The average oxide composition of the filter, determined byX-ray fluorescence, was 27% silica, 68% alumina and 4% magnesia. Thecrystalline phase composition, determined by X-ray diffraction, was 33%cordierite, 8% mullite, 49% corundum and 10% cristobalite.

Example 3

This example illustrates the fabrication of a ceramic filter of thecurrent invention, wherein the membrane layer is formed by multiple yarnwinding. A filter support element was prepared as described in Example1.

The membrane layer was formed using the same filament winder as was usedto form the support. Yarns from three different bobbins of S-2 CG 1501/2 636 glass yarn were combined and fed through a tension device,dipped in the A-17/fumed alumina slurry described in Example 1, pulledthrough a 0.025 in (0.64 mm) diameter sizing orifice, and wound on thesupport. The same wind angle, mandrel rotation rate, and traverse armspeed used for the support was used for winding the membrane layer. Thewinding was continued until two layers of yarn had been wound onto themandrel so that the yarn covered the entire surface of the support.After drying overnight, the bottom end and flange sections were treatedas described in Example 2. The assembly was then high temperature firedas described in Example 1.

The membrane layer had a bulk density of 1.75 g/cc and a volume porosityof 37% with a median pore diameter of 0.64 μm, as measured by mercuryporosimetry. The average oxide composition, determined by X-rayfluorescence, was 27% silica, 68% alumina and 4% magnesia. Thecrystalline phase composition, determined by X-ray diffraction, was 35%cordierite, 6% mullite, 50% corundum and 9% cristobalite.

What is claimed is:
 1. A method of making a ceramic hot gas filtercomprising:(a) fabricating an elongated porous filter support having anouter surface, an opening at one end into a hollow interior, a closedend opposite said open end, an external flange integral with said openend by coating a ceramic oxide support yarn with a first oxide ceramicmaterial, winding said coated ceramic oxide support yarn onto a mandrelto form a plurality of layers of said coated support yarn, each layerbeing arranged in a criss-crossing relationship with neighboring layersto form a plurality of quadrilateral-shaped openings wherein saidwinding is controlled to provide dimensions of said quadrilateral shapedopenings between about 100 to 500 microns after the heat treatment andsaid support having an outer surface and openings at its ends into ahollow interior, said mandrel being contoured to provide an externalflange as an integral part of said support adjacent one of saidopenings, said first oxide ceramic material providing, upon heattreatment, a porous refractory oxide support matrix; (b) drying thesupport then removing the mandrel; (c) forming a porous membrane layerhaving a porosity less than that of said support by coating a continuousfilament oxide ceramic membrane yarn with a second oxide ceramicmaterial, applying said coated membrane yarn in an ordered arrangementon the outer surface of said support using a method selected from thegroup consisting of hoop winding, multiple yarn winding, and fabricwrapping, and wherein said second oxide ceramic material providing, uponheat treatment, a porous refractory oxide membrane matrix; (d) closingthe end of said support opposite said flanged end with an oxide ceramic;and (e) heat treating said support and said membrane layer.
 2. Theprocess of claim 1 wherein said second oxide ceramic material is coatedon said membrane yarn in a sufficient amount to provide membrane porediameters of 0.1 to 10 microns.
 3. The process of claim 2 wherein saidsupport yarn and said membrane yarn each comprise at least 20 weightpercent alumina and have a softening point above 750 degrees C.
 4. Theprocess of claim 2 wherein said support yarn has generally the samecomposition as said membrane yarn and wherein said support matrix hasgenerally the same composition as said membrane matrix.
 5. The processof claim 2 wherein said first oxide ceramic material and said secondoxide ceramic material each comprise an aqueous slurry of a ceramicoxide particulate.
 6. The process of claim 5 wherein said ceramic oxideparticulate in the aqueous slurry consists essentially of alumina. 7.The process of claim 6 wherein said support yarn and said membrane yarneach comprise between about 61-66 wt % SiO₂, 24-26 wt % Al₂ O₃, and 9-10wt % MgO.
 8. The process of claim 5 wherein said first oxide ceramicmaterial and said second oxide ceramic material each further comprise aceramic oxide precursor.